This is for OS X.
I first went to here and downloaded the Eclipse classic version. I also downloaded the 64-bit one.
Another couple of tidbits...
Using MAMP -> Other MAMP here
First thing you do is extract it. I then moved the Eclipse folder to /Applications
I then launched Eclipse. It told me I didn't have Java. It will launch Software Update and download the correct version of Java.
It will then ask you about setting up a workspace. If your using MAMP and want to use the htdocs folder, you will want to set the workspace to /Applications/MAMP/.
Note: In the instance you setup the workspace to some where else, go to "File -> Switchwork Space -> Other" to change it.
Next step is to go to Help -> Install New Software. Select the drop down box next to "Work with:" and select "--All Available Sites --". Wait a few minutes while it populates the fields below. In the filter box type PHP and you'll get 3 choices to install. Click finish and they will install.
If you already have existing files in a folder your going to use, I suggest as a backup to do a zip of all the files in the event something goes wrong. I was fine, but just to be on the safe side, do it!